Associated Press photosUnited States Secretary of Homeland Security Janet Napolitano speaks during a swearing-in ceremony for 5,000 new citizens at Fenway Park in Boston, Tuesday.

BOSTON – Homeland Security Secretary Janet Napolitano welcomed more than 5,000 new citizens in a ceremony Tuesday at Boston’s Fenway Park, in what officials called one of the largest naturalization ceremonies in the nation’s history.

Speaking to immigrants from roughly 150 countries, Napolitano congratulated the new citizens for their “long journey” toward citizenship but told them that they now faced new civic responsibilities as Americans.

“America is now your country,” Napolitano said to an audience of an estimated 15,000. “I hope you take an active role in you community and continue learning about our history.”

It was the second time that the home of the Green Monster and the Boston Red Sox – who were in Seattle to play the Mariners – was transformed into a federal court and used for a naturalization ceremony.

In 2008, the Oath of Citizenship was taken at Fenway Park by about 3,000 immigrants.

Elisangela Sherman cheers as people from Brazil, her country of origin, are invited to stand during a swearing-in ceremony for 5,000 new citizens at Fenway Park in Boston, Tuesday.

Thu Bich Le, 31, of Randolph said she was excited to take the oath with her mother, 53-year-old Chau Phan. Le said both came from Vietnam 21 years ago.

“America is my home,” said Le, who works at a Japanese restaurant. “I’m ready to vote.”

Sgt. Monique Dini, 33, came to the ceremony on a two-week leave from Kuwait to surprise her 60-year-old mother, Els Dini, of Boxford. Els Dini, from the Netherlands, did not know her daughter was sitting in the audience until an official announced it to the crowd.

“Basically, I just got off the plane,” said Monique Dini, who serves in the National Guard. “I wanted to give her a surprise.”

After the ceremony, Napolitano told reporters that holding such a large citizenship oath event in Boston was an ideal location given the region’s history with the first migration from Europe.

“How great is it to have it in Boston, where our country was founded in part?” said Napolitano.

She sidestepped questions about illegal immigration and Arizona Sheriff Joe Arpaio’s recent visit to Nashua, N.H., for a GOP fundraiser. Arpaio, who is known for his efforts against illegal immigration, is under federal investigation for potential civil rights violations.